This invention relates generally to semiconductor structures and devices and to a method for their fabrication, and more specifically to compound semiconductor structures and devices and to the fabrication and use of semiconductor structures, devices, and integrated circuits that include a monocrystalline compound semiconductor material. More specifically, this invention relates to semiconductor structures for use with microwave and radio-frequency (RF) signals, and to a method for their fabrication.
The vast majority of semiconductor discrete devices and integrated circuits are fabricated from silicon, at least in part because of the availability of inexpensive, high quality monocrystalline silicon substrates. Other semiconductor materials, such as the so called compound semiconductor materials, have physical attributes, including wider bandgap and/or higher mobility than silicon, or direct bandgaps that makes these materials advantageous for certain types of semiconductor devices. Unfortunately, compound semiconductor materials are generally much more expensive than silicon and are not available in large wafers as is silicon. Gallium arsenide (GaAs), the most readily available compound semiconductor material, is available in wafers only up to about 150 millimeters (mm) in diameter. In contrast, silicon wafers are available up to about 300 mm and are widely available at 200 mm. The 150 mm GaAs wafers are many times more expensive than are their silicon counterparts. Wafers of other compound semiconductor materials are even less available and are more expensive than GaAs.
Because of the desirable characteristics of compound semiconductor materials, and because of their present generally high cost and low availability in bulk form, for many years attempts have been made to grow thin films of the compound semiconductor materials on a foreign substrate. To achieve optimal characteristics of the compound semiconductor material, however, a monocrystalline film of high crystalline quality is desired. Attempts have been made, for example, to grow layers of a monocrystalline compound semiconductor material on germanium, silicon, and various insulators. These attempts have generally been unsuccessful because lattice mismatches between the host crystal and the grown crystal have caused the resulting thin film of compound semiconductor material to be of low crystalline quality.
If a large area thin film of high quality monocrystalline compound semiconductor material was available at low cost, a variety of semiconductor devices could advantageously be fabricated in that film at a low cost compared to the cost of fabricating such devices on a bulk wafer of compound semiconductor material or in an epitaxial film of such material on a bulk wafer of compound semiconductor material. In addition, if a thin film of high quality monocrystalline compound semiconductor material could be realized on a bulk wafer such as a silicon wafer, an integrated device structure could be achieved that took advantage of the best properties of both the silicon and the compound semiconductor material.
One application for such a compound semiconductor is in devices that use microwave or radio frequency (RF) signals. Such devices include, for example, intelligent transportation systems such as automobile radar systems, smart cruise control systems, collision avoidance systems, crash protection systems and automotive navigation systems. They also include electronic payment systems that use microwave or RF signals such as electronic toll payment for various transportation systems including train fares, toll roads, parking structures, and toll bridges.
Largely in an effort to reduce high cost to society of automobile accidents caused by driver error, collision avoidance systems have been proposed that typically use radar installed on automobiles to detect objects in the path of the automobile and warn the operator, apply the vehicle brakes, or take other evasive action when necessary to avoid a collision. In addition to monitoring objects directly in an automobile""s path, collision avoidance systems can preferably monitor other vehicles in adjacent lanes in case those other vehicles wonder into the automobile""s lane. They may also be used to detect objects on the side of the automobile; for instance, to warn a driver not to change lanes when another vehicle is in his blind spot. Collision avoidance systems have also been proposed to detect objects behind the automobile; for instance, for use when backing up to warn a driver that he is about to hit another vehicle or run over a small child.
Crash protection systems have also been proposed which would detect an imminent collision and take appropriate action. This action may include deploying air bags prior to a collision actually occurring. Such a crash protection system would allow more time for air bags to deploy, thus reducing the required force associated with their deployment. This could reduce the potential for injury resulting from air bag deployment, particularly to small children. It has been proposed that crash protection systems may also increase the effectiveness of air bags in particular types of collisions, such as side air bags deployed in side impact collisions. Pre-impact deployment would be a particular advantage is such applications where the distance between an occupant and the structure of the automobile is already marginally sufficient for deployment of an air bag.
Inventors have further proposed that such radar systems could tie into the automobile""s cruise control system, and automatically slow the speed of the automobile when traffic conditions warrant, rather than relying the driver to perform this function. Such systems are called intelligent cruise control systems, and in addition to improving safety, also relieve some of the stress associated with operating an automobile in traffic. Rather than being limited to maintaining a constant speed, when traffic prevented traveling at the desired maximum speed, intelligent cruise control systems would automatically adjust the speed of the automobile to maintain a safe distance between it and the vehicle in front of it. If manipulation of the throttle was not sufficient, the system would preferably automatically apply the automobile""s brakes. If widely used, intelligent cruise control systems may also have other advantages. For instance, such systems could be programmed to maintain more constant speeds in traffic situations. This could reduce fuel consumption and vehicle wear. Also, if reaction times are quicker and more reliable than those of human drivers, then distances between automobiles could be reduced, thus increasing the carrying capacity of roadways and increasing the average speed of traffic.
Collision avoidance systems and intelligent cruise control systems require, inter alia, some kind of a detector mounted at the front of the automobile called a forward-looking sensor. Forward-looking sensors are typically radar systems such as continuous-wave radar, two-frequency continuous-wave radar, or frequency-modulated continuous-wave radar. As the name implies, continuous-wave radar continuously transmits a signal at a single frequency. The transmitted energy is reflected by objects in its path and received by a radar receiver. The frequency of the received signal is slightly changed (Doppler shifted) by the movement of the object relative to the automobile. By detecting the change in frequency, the system is able to determine the speed of the object relative to the automobile. However, a single frequency continuous wave radar system is not able to determine how far away the object is from the automobile. A two-frequency radar system transmits a signal at two frequencies and looks at the phase difference between the two returning signals as well as the Doppler shift. From this, the distance to the object, as well as the relative speed, can be calculated. Frequency-modulated continuous-wave radar continuously changes the frequency of the signal, back and forth over a range of frequencies. By looking at the frequency received from an object, the system can determine when the signal was transmitted; and hence, how far away the object is. Forward-looking sensors may also be multibeam radar systems. Other types of radar are also well know to a person of ordinary skill in the art of radar systems.
Automobile navigation systems have been proposed that transmit navigation information to automobiles traveling along a highway. The navigation information that is transmitted typically includes the position of the automobile, and the driver of the automobile is typically given instructions regarding what route to take, such as how to reach a destination he has entered into the system. The driver is preferably detoured around or warned about dangerous or congestive traffic conditions. Automobile navigation systems preferably also use a global positioning system (GPS) to determine the location of the automobile, and may be combined with collision avoidance systems, intelligent cruise control systems, and/or automatic toll collecting systems.
Electronic payment systems are in use, or have been proposed, that involve the identification of an automobile, smart card, or other object for the purpose of paying a fee. Some such systems do not require physical contact with the object, or even close proximity (such as a to read a magnetic strip), but rather, through the use of a radio frequency (RF) or microwave signal, only require a general proximity. Such systems associated with automobiles more commonly use RF signals, and include toll payment for toll roads or toll bridges, and parking fee payment for entering a parking garage. Such systems may alternatively use laser (bar code) readers or infra-red (IR). These systems generally include an interrogator (reader) located in the toll booth, and a transponder (tag) installed on the dashboard, windshield or other location on the automobile. The interrogator may, for example, send a signal that triggers the transponder, interrogate the transponder for specific information, and provide an acknowledgement message to the transponder. The tag typically identifies the automobile (via an identification code), providing sufficient information to bill, debit, or charge the owner for the amount of the toll. Tags may contain only read-only information, or may (smart tags) hold information as to the balance in the user""s account. This communication can typically all take place while an automobile drives through the toll booth without stopping.
Such a system typically must have some kind of enforcement means for deterring passage through the toll booth by individuals who do not have transponders or are delinquent in their payment of tolls. Such enforcement means may include a gate that opens only when valid account information has been provided, or use of a surveillance camera that photographs offenders to collect evidence for bill collection or criminal enforcement.
Electronic toll collection eliminates the need for commuters to stop at toll booths, and eliminates the need for them to carry cash or other means of paying tolls. These consequences have various direct and indirect benefits including reducing commute times, reducing the need for personnel to man toll booths, reducing fuel consumption and air pollution caused by commuters stopping and waiting in line at toll booths, and reducing crime resulting from the temptation of commuters carrying cash to pay tolls. Similar systems have been proposed or are being used to pay tolls for riding mass transit systems such as trains, or for purchasing various commodities such as gasoline. Tags for such systems may be read-only, or may contain information such as regarding account balance (smart cards).
Today""s systems for automobile radar or communication between an automobile and a fixed location typically use a hybrid of gallium arsenide (GaAs) or indium phosphide (InP) devices for sensing signals, and silicon (Si) based devices to do the signal processing and direct the appropriate signal to the appropriate circuitry. Most commonly, GaAs monolithic microwave integrated circuits (MMICs) are used for sensing, and a separate Si chip performs the signal processing. These MMICs contain both active elements such as diodes and transistors, and passive elements such as resistors, capacitors, and inductors, and have replaced earlier more-conventional xe2x80x9cchip and wirexe2x80x9d microwave and RF circuits. However, it would further simplify the structure and reduce the cost to be able to perform both functions (signal sensing, and signal processing) with one integrated circuit. In addition, as discussed above, compound semiconductors are very expensive, which has been a significant factor in preventing the above described systems, with their associated benefits to safety, reduction of energy consumption, collateral cost savings, and reduction of environmental pollution, from being widely applied. Accordingly, a need exists in applications that use high-frequency signals, for an inexpensive semiconductor structure that provides a high quality monocrystalline compound semiconductor film over another monocrystalline material and for a process for making such a structure.